Participants:

Go to the profile, select the picture (from their list of pictures) of the one you want to make the profile picture. At the top, you'll see an option to make it their profile photo. Just be careful, depending on how you get to the photo, sometimes it can say make this "your" profile photo. If you get to it through the other persons profile (the one you want to set), it should give the option to make it their profile photo.